Formation of lava tubes requires a special kind of
volcanic eruption: those where fluid lava flows down the sides. The
more explosive type of eruption known as pyroclastic, where rocks, cinders,
and ash are ejected, donít form tubes. Within fluid lava flows, there
are two main mechanisms for forming tubes. In one type, the upper surface
of the lava flow begins to cool, and the lava beneath continues to flow
in tubular conduits beneath the surface. In another common scenario,
channels carrying lava become roofed over as material accretes to the
side, eventually forming a solid roof.

Due to the insulating effects of the hardened lava
above, molten lava is able to travel considerable distance underground
with very little cooling. In Hawaii, lava tubes have carried fluid lavas
50 or more miles from their source and thereby played a large role in
shaping the islands.

Whether on the surface or underground, flowing lava
behaves like most rivers would, flowing downhill, around obstructions.
Lava flowing underground sometimes returns to the surface through existing
skylights, or ruptures the surface when
the flow volume becomes too high, creating a breakout entrance.

All the photos on this page, except for the top one,
were taken by scientists working for the U.S. Geological Survey based
at the Hawaiian Volcano Observatory, and are used by permission of the
USGS.

Above: Skylights indicate an active tube
flowing under a hardened crust, and below, a closer view of an active
skylight.

Above: Channelized lava with a large piece
of hardened crust wedged in the center, which is one way in which a channel
can roof over to form a tube. Below, another mechanism for a channel to
roof over is by accretion of material along the edges that can coalesce
to form a roof.

Choose a thumbnail above to zoom in on more images
of active skylights from the
USGS archives.